Frequency synthesizers are commonly used in communications systems, as well as other applications. The oscillators of the frequency synthesizers are often implemented as LC tank oscillators, which include inductors and capacitors arranged to oscillate by exchanging current or voltage between inductors and capacitors with a finite frequency.
Ideally, an oscillator should have no loss and should be capable of oscillating at the same frequency forever. In most cases, however, the inductor in an LC tank oscillator is non-ideal and has some resistance that causes energy loss and instability in oscillation. The stability of the oscillation, as measured by the quality factor, Q, is proportional to the energy stored in the LC tank and is inversely proportional to the energy dissipated in the resistor per oscillation cycle. As the resistance of the inductor increases, Q decreases and the oscillator introduces more instability to the system. The phase noise added to the system also increases. Applications such as the frequency synthesizers used in cellular telephones require low phase noise, and hence high Q oscillators are suitable for these applications. The frequency synthesizers used in these applications also need to be highly tunable to meet the demands of the cellular telephony standards.
While it is desirable to integrate the inductor onto the same chip as the frequency synthesizer, attempts to build an on-chip inductor using conventional deposition techniques have mostly resulted in low Q oscillators due to the resistance of the material. Certain other techniques, such as constructing the inductor using microelectricalmechanical systems (MEMS) technology, can be used to produce integrated oscillators with good Q values but tend to be expensive and hard to mass-produce. Thus, many of the systems that require high Q oscillators leave the inductor off-chip.
FIG. 1 is a block diagram illustrating a frequency synthesizer with an off-chip inductor. The input of frequency synthesizer 100 is a signal with a reference frequency. The frequency synthesizer includes a Phase locked loop (PLL) 105 and a voltage controlled oscillator (VCO) 110. For the purpose of clarity, the rest of the details of the frequency synthesizer, PLL and VCO are not shown. The PLL is configured to apply a voltage to the VCO, which then outputs a signal with a desired frequency. The signal output by the VCO is the output of the frequency synthesizer. VCO 110 has an LC tank oscillator with an inductor 120 that is external to the frequency synthesizer chip. The inductor may be an external component bonded to the chip via external pins or bond wires, or a copper trace deposited on a printed circuit board. There are several disadvantages to using an external inductor, including high sensitivity to noise and complexity of packaging.
It would be useful to have frequency synthesizers that are highly tunable. To achieve noise resistance and reduce packaging cost, it would be desirable to have frequency synthesizer designs with low phase noise and high Q oscillators that have on-chip inductors. Furthermore, it would also be desirable to be able to produce these frequency synthesizers using conventional integrated circuit manufacturing techniques.